Echoing what someone else has said, when it’s something that’s so unknown or ill-understood, just denying it could work, but probably only if you’re not visibly disabled.

Have you tried asking why they do that? Has that ever worked? “Why did you tell that person that I have autism/anything?”

realsocialskills said:

That’s another strategy that can sometimes work but that can also backfire horribly. Sometimes if you phrase things as a question, it makes the other person feel heard and less defensive. That doesn’t always work, but it can sometimes.

That sounds horribly obvious, but have you tried asking? “Mom, I would like to decide on my own if or when I tell people about my autism [or other reasons]. Can you please in the future refrain from talking about it with new people you meet?” It’s true, you can’t actually “make” people stop anything. Sometimes asking helps, sometimes people stay inconsiderate :/ If you think you know the motivation behind it, you can also propose other topics of conversation instead. E.g. “It’s nice you want to tell people about me, how about instead you talk about [a favourite activity of mine].”

When I first got officially diagnosed with possible autism in my late teens (circa 2000), I explained to my mother that there was a bunch of really scary, stigmatizing, dehumanizing misinformation going around about autism, and that many autism advocacy groups were focused on using fearmongering to raise funds for dangerous quackery. I told her that she wasn’t allowed to tell anyone without my permission, not even family members, because I didn’t want people to gossip about it. She respected my decision and hasn’t outed me to anyone.